§H.12 step 13: blk_meta_t flags bit constants

BLK_FLAG_CLAIMED/BLK_FLAG_MIGRATING/BLK_FLAG_STALE (bits 0/1/2), matching
the decided §F.4/§H.6 layout. Orthogonal bits, not a mutually-exclusive
enum.

Verified 3-arch boot to ok> (amd64/aarch64/riscv64).

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>

include/block_subsystem.h

@@ -199,6 +199,16 @@ typedef struct {
 _Static_assert(sizeof(blk_volume_meta_t) == 4096,
                "blk_volume_meta_t must be exactly one 4 KiB devblock");
 
+/* blk_meta_t.flags bit values -- FABRIC-3.md §F.4/§H.6/§H.12 step 13,
+ * decided 2026-09-02/03. Orthogonal bits, not a mutually-exclusive enum:
+ * a block can be both CLAIMED and MIGRATING at once. Grounded in the only
+ * states §F.4 actually motivated by a real need (MIGSM/UNCLEAN, two
+ * then-currently-blocked graph nodes) plus CLAIMED/STALE, the names
+ * already used loosely in that pass's own prose. 61 bits remain reserved. */
+#define BLK_FLAG_CLAIMED   (1ull << 0) /* owned, per BMAPFMT's owner_fp */
+#define BLK_FLAG_MIGRATING (1ull << 1) /* mid-migration; serves MIGSM */
+#define BLK_FLAG_STALE     (1ull << 2) /* interrupted flush; serves UNCLEAN */
+
 /* Per-1 KiB block metadata (packed into top 1 KiB region of each 4 KiB sector). */
 typedef struct {
     /* Core integrity (16 bytes) */
@@ -210,7 +220,7 @@ typedef struct {
     uint64_t modified_time; /* Unix timestamp (last write) */
 
     /* Block status (16 bytes) */
-    uint64_t flags; /* Status flags */
+    uint64_t flags; /* Status flags -- BLK_FLAG_* bits above */
     uint64_t write_count; /* Number of writes (wear leveling) */
 
     /* Content identification (32 bytes) */